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A stimulant found naturally in coffee beans, tea leaves, cocoa beans (chocolate) and kola nuts (cola) and added to soft drinks, foods, and medicines. A cup of coffee has 100-250 milligrams of caffeine. Black tea brewed for 4 minutes has 40-100 milligrams. Green tea has one-third as much caffeine as black tea.

Caffeine is an alkaloid. It is metabolized in the liver and the breakdown products of caffeine are excreted through the kidney. In women on oral contraceptives, the rate at which they clear caffeine from the body is considerably slower. Pregnancy reduces a woman's ability to process caffeine still further. The half life of caffeine in an adult is about 3 to 4 hours. In pregnancy, it is 18 hours.

In doses of 100-200 mg. caffeine can increase alertness, relieve drowsiness and improve thinking. At doses of 250-700 mg/day, caffeine can cause anxiety, insomnia, nervousness, hypertension, and insomnia. Caffeine is a diuretic and increases urination. It can curiously enough make it more difficult to lose weight because it stimulates insulin secretion, which reduces serum glucose, which increases hunger.

Caffeine can help relieve some headaches, so a number of over-the-counter and prescription pain relievers include it as an ingredient, usually with lymphedema and pain managementaspirin]] or another analgesic.

The word “caffeine” came from the German Kaffee and the French café meaning, of course, coffee.